Isolation of vesicular stomatitis virus from mouse mothers after inoculation of suckling mouse litters.

with specific toxic effects on the spreading elements of the organisms. The purpose of this note is to describe the transformation of strains of Proteus mirabilis to the L form of growth under the influence of sterile broth filtrates of antagonistic strains. The test organisms used in this study were selected by streaking opposite poles of nutrient agar plates with recent isolates of Proteus mirabilis. Antagonism between two strains was evidenced by a line of demarcation between the spreading elements of the organisms. The L type of growth was induced in nutrient agar pour plates containing high concentrations of filtrates from 10-day-old broth cultures of a selected strain in conjunction with an appropriate test organism. The L colonies were similar to the 3B L colonies obtained by Dienes (J. Bacteriol., 57, 529, 1949) with the use of penicillin. In most cases L transformation was not complete insofar as smooth, nonswarming colonies of the test organism were also present. The L forms could be maintained in subculture only in the presence of the filtrate. Reversion to the normal bacillary state was easily accomplished by streaking the L forms on filtrate-free plates. The reverted organisms were identical with those of the parent strain and showed no increased resistance to the action of the filtrate. Repeated cultivation in the presence of the filtrate did not slow down the process of reversion or increase the resistance to the filtrate. The metabolic activity of the L forms as evidenced by biochemical characteristics was found to be very similar to the parent organisms. Microscopic observations of the transformation of bacilli to large bodies and L forms in slide cultures showed processes similar to those reported in other investigations (Dienes, J. Bacteriol., 57, 529, 1949; Stempen and Hutchinson, J. Bacteriol., 61, 321, 1951). Dienes (personal communication) indicated that it was doubtful that the action between the spreaders and the inhibitory action of old broth cultures depended on the same mechanisms, since the antagonism between the spreaders is specific and does not affect the small bacterial forms. Preliminary data indicate that several factors may be present in filtrates which induce L transformation. Furthermore, we have demonstrated that the filtrate also induces L transformation on its homologous source. However, the latter effect is not as marked as that which occurs against the heterologous strains. Although many conditions that induce L transformation are artificial, it is felt that the observation that Proteus cells can give rise to a spontaneous product of growth capable of inducing L transformation would seem to indicate that such a process might occur in nature. The observation also lends support to the statement by Dienes and Weinberger (Bacteriol. Revs., 15, 245, 1951) that L transformation may be the result of general injury to normal bacterial growth rather than a specific effect of a specific toxic substance.